In telecommunication networks such as fixed access networks and cellular radio telephone networks there is usually a need to record call information at at least one exchange of the network. This information may include the identity of a caller (A-number), the identity of the called party (B-number), and the duration of a call. In cellular radio telephone networks, the recorded information may also include the International Mobile Subscriber Identity (IMSI) code of the Subscriber Identity Module (SIM) used with the calling mobile telephone, and the International Mobile Equipment Identity (IMEI) code of the calling telephone itself. Both the IMSI and IMEI codes are typically sent by a mobile telephone to the cellular network during a call set-up phase. For a given call, a record stored in an exchange is normally output to a centralised billing system of the network upon termination of the call. Historically, this record has been referred to as a “Toll Ticket” (TT) although more recently the term “Call Data Record” (CDR) has been used.
U.S. Pat. No. 5,506,893 describes a telecommunication network in which a CDR is output from a switching centre to an external billing system upon termination of a call.